This invention relates to a method and a device for exposing a sheet. More particularly, this invention relates to a method and device for exposing both sides of a sensitized sheet for direct imaging of printed circuit board panels, printing plates, or other sensitized sheets.
One aspect of this invention is exposing a sheet that is provided with a sensitive layer, for example a light-sensitive layer on one or both sides (i.e., on the surface or surfaces of one or both sides). A light-sensitive layer herein includes a layer sensitive to thermal (e.g., infra-red or IR) radiation, visible light, and ultra-violet (UV) radiation.
One application of the invention is the direct imaging of a single or double-sided sensitized sheet for producing a printed circuit board (PCB). Other applications will be clear from the following detailed description.
It is known that printed circuit boards may be composed of several PCB panels, each of which is provided with an electrical circuit. When there are only two layers, the board is commonly called a double-sided board, and when there are more than two layers, the board is commonly called a multi-layer board. A common way of manufacturing a multi-layer board is by fixing several panels together, each panel having a single printed circuit on one side, or a circuit on each side. xe2x80x9cOuterxe2x80x9d panels are those that face the outside of a multi-layer PCB, and xe2x80x9cinner panelsxe2x80x9d are the interior panels. Typically, the inner panels have a circuit on both sides, while the outer panels have a circuit only on one, the outer side. Each inner panel resembles a thin double-sided PCB in that the panel is comprised of an insulating substrate which is clad on both sides with metallic foil, typically copper foil. A printed circuit is formed on any circuit side of an inner panel by that side""s metal cladding having a light-sensitive layer laid on top of the metal. The light-sensitive layer is exposed to light (typically ultra-violet (UV) radiation) at selected locations, then processed by a photographic process that removes the layer at selected locations. An etching process is then applied to remove those parts of the layer of metal not necessary for forming the actual circuit. Once all the double-sided inner panels are produced, they are fused (pressed) together by placing an insulating binding material, typically a partially cured epoxy-resin material called prepreg, between the panels. Unexposed outer foils are placed on the outside of the double-sided inner panels, again with prepreg in between. All the layers are now laminated by applying heat and pressure that causes the prepreg to flow and bond to the surfaces of the inner panels and the outer foils. Holes are now drilled on the laminated multi-layer board, including holes for mounting electrical components inserted into the board (called xe2x80x9cmounting holesxe2x80x9d), and holes for making contacts from one layer to one or more other layers (feed-throughs, also called vias or conductive vias). The holes typically are plated through. Each side of the multi-layer panel now is sensitized, then exposed and processed to form the two outer printed circuits in exactly the same manner as forming circuits on the inner panels.
Note that because a multi-layer panel is exposed in the same way as an inner PCB panel, the words xe2x80x9cPCB panelxe2x80x9d or simply panel will mean either a complete PCB board, an inner PCB panel, or a post-lamination multi-layer panel.
One difficulty in producing multi-layered printed circuit boards is the strict requirement for accuracy in positioning the different PCB panels together to ensure that the different circuits are positioned very accurately relative to each other. In particular, the mounting holes and vias need to be very accurately placed on each layer""s circuits. For a particular tolerance for the placement of a circuit, it is clear that any deviations in the specified location of the circuits on each of the layers may be additive, so that at any one location, there could be large deviations. For the case of double-sided panels, including the multi-layer panel after lamination, it is even more difficult to position the circuits accurately enough relative to each other.
A common method for producing printed circuit boards is to first produce artwork, which is an accurately scaled configuration used to produce a master pattern of a printed circuit, and is generally prepared at an enlarged scale using various width tapes and special shapes to represent conductors. The items of artwork, once reduced, for example, by a camera onto film to the correct final size, are referred to as phototools and are used as masks for exposing the sensitized layers. Because the photographic reduction is never 100 percent accurate, more accurate phototools are produced nowadays using photoplotters rather than photographic reduction.
However produced, physical phototools are susceptible to damage. In addition, whenever any amendments need to be made to any circuit, new phototools need to be produced. Furthermore phototools, sometimes in the form of photographic negatives, are difficult to store. They also may not be stable; their characteristics might change with temperature and humidity changes.
There thus are advantages to directly imaging the required circuit patterns onto PCB panels, for example PCB panels that include a light-sensitive layer on one or both sides. The same advantage also is applicable to directly imaging printing plates that include a UV, visible light, or thermally- sensitive layer. Often such sensitive sheets as used for PCBs or thermal printing plates are rigid, so that the scanning apparatus for exposing such sheets for direct imaging (e.g., directly exposing printing plates or directly exposing PCB panels) is of the flat-bed type in which the sheet is disposed on a horizontal table for exposure by the light energy (e.g., UV light or infrared) produced by the scanner. Such scanning apparatuses are typically quite bulky because of the horizontal table. Also, such direct imaging systems expose one side at a time, and there are problems accurately aligning the two sides for double-sided exposure.
Thus there is a need in the art for an improved method and apparatus for exposing a sheet, one or both sides of which are provided with a light-sensitive layer, which may be a UV sensitive layer, a visible light-sensitive layer, or a thermally sensitive layer.
In particular, there is a need in the art for a method and apparatus for reducing the probability of differences occurring between the required locations of the desired scanned regions (e.g., the images) at the respective sides of the sheet. In the case of the sheet being a PCB panel, it is desired to reduce the probability of there being unwanted differences in locations of the printed circuits at the respective sides of the PCB panel.
Described herein are a method and an apparatus for exposing a light-sensitive sheet, in which the exposing is carried out by one or more optical beams projected onto the respective light-sensitive layers, with the particularity that the scanning is carried out from two opposite sides of the sheet to be scanned. Optical herein includes in a non-limiting way thermal radiation such as infra-red, UV and visible light.
In a preferred embodiment the exposing operations at both sides are carried out simultaneously. Furthermore, in a particular embodiment, the exposing operations are carried out by scanning simultaneously at the same portions of each side of the sheet. That is, when carrying out a scanning action at a particular region at one side, the scanning of the corresponding region at the opposite side of the sheet is done at substantially (and not necessarily exactly) the same time.
In the preferred embodiment, only one energy source, for example, a UV light source, is used for scanning both sensitive layers, this arrangement having the advantage that the resulting device for carrying out the method can be relatively compact. A further advantage of using one energy source for both sides is that the cost is lower than if more than one source is used. Using the same source for exposing both sides raises the system""s optical efficiency, so that the laser power requirements may be lowered. Other embodiments may include more than one energy source.
One advantage of exposing the sheet on either or both sides is that the scanning can be carried out in a shorter amount of time than if one needed to present the sheet twice to the scanning apparatus. Scanning both sides simultaneously further reduces the scanning time.
When exposing both sides at the same time, the relative positioning of the scanned regions (i.e., the images) created at the respective sides is less sensitive to any panel or sheet changes or distortions that may occur over time. Furthermore, exposing both sides in a device that includes a mutual positioning mechanism assures that the images of the two sides are positionally accurate with respect to each other.
In one embodiment, optical exposing units are used for exposing the sides of the sheet by the energy of the one or more energy sources, and these optical exposing units may be positioned opposite each other, either in front of each side of the sheet or offset therefrom. In one embodiment, the optical exposing units are optical scanning units which form one or more optical beams which form scan lines on each side, while in another embodiment, applicable to exposing the sides of the sensitive sheet according to imaging data, the optical exposing units are imaging units that project sub-images (xe2x80x9ctilesxe2x80x9d) according to the imaging data. Complete images may be exposed for example in a step and re-image manner in which new sub-images are sequentially imaged after a stepping motion to the new sub-image location, each placement of a new sub-image using the imaging data corresponding to the new sub-image. Alternate flat-bed imaging methods similarly may be used together with a cross-positioning mechanism for accurately positioning the two sides relative to each other. Another aspect of the present invention is a method and apparatus for adjusting the scanning movement of the beam or beams of each optical scanning unit or the sub-images of each optical imaging unit relative to each other, so that the sets of scan lines or sub-images covering the desired regions generated by both units are automatically mutually positioned. In one embodiment of the adjustment method applied in the beam forming case, the beam of at least one of the scanning units is sensed by means of a detecting module coupled to the other scanning unit. That is, in the preferred embodiment, the beams produced by both optical scanning units are sensed by a single detection module. In this way perfect registration is substantially obtained. The particular embodiment of the detection module makes such mutual adjustment possible, irrespective of the thickness of the sheet to be scanned, within some dimensional limits. Other embodiments include using more than one optical detection module to detect the beams produced by both optical scanning units.
According to another aspect of the invention, an optical detection module is used which detects the cross-scan position of the beam (both beams in the case of the mutual positioning system) in a manner that is insensitive to the focus of the beam, using two photodiode elements side by side so that the difference in the beam path traversed through each diode is indicative of the cross beam position. Several embodiments of such a detection module are described with the shape of the surfaces providing different desirable characteristics. In some of these embodiments, the two diodes also provide an indication of when a beam traverses a fixed point in the scan beam direction. In other of these embodiments, an additional photodiode element is provided and the interface of such a third photodiode with one of the other two photodiodes provides for indicating when a beam crosses a given location in the scan line direction.
According to yet another aspect of the invention, the above-mentioned object of exposing both sides of a sheet is obtained by providing a method wherein the exposing is done by means of only one exposing unit located at one side, whereas each side of the sensitive sheet, which may consist of a plate, board or PCB panel, is presented successively to the optical exposing unit, the sheet being fixed in a carrier from the beginning of the exposing (e.g., by scanning by an optical scanning unit) of the first side until the end of the exposing of the second side. According to this embodiment, the sheet remains fixed in the carrier throughout the exposing of both sides. Because such a carrier can be positioned with high accuracy in relation to the optical exposing unit, and because the sheet does not need to be removed from the carrier until the scanning of both sides is complete, it is clear that the images and consequently the printed circuits can be positioned in respect to each other very exactly.
In this single optical exposing unit embodiment, the carrier and the single optical exposing unit may be mutually movable in such a manner that the sheet carried by the carrier can be exposed successively at both sides by the optical exposing unit, for example by scanning using an optical scanning unit. According to an alternative, the carrier may consist of a holder which can be positioned in either two positions in a support mechanism for this holder. The sheet is first fixed in the holder and, in order to expose both sides of the sheet, the holder is first placed into the support mechanism in one orientation. The first side is exposed, and then the holder is removed, turned, and then re-inserted into the support mechanism for the scanning of the second side.
The present invention also relates to a method for scanning a sensitive sheet to be used for a PCB panel, the method being characterized in that during scanning, the sheet is positioned in an upright position. Scanning can be performed from both sides simultaneously, or can be done only from one side. One advantage of this method is that the method can be realized with relatively compact devices, as a horizontal table is no longer required.
The upright construction also provides better accessibility to the optical exposing units. More particularly, in the case of optical scanning units, the arrangement provides good access to the optical heads that are included in each optical scanning unit. Furthermore a completely symmetrical lay-out is possible.
The present invention also relates to a method for exposing a light-sensitive sheet such as a light-sensitive PCB panel, the method being characterized in that the image of the printed circuit on one or both sides is formed by direct imaging. In this way disadvantages in using phototools such as negatives are excluded.
The present invention also relates to an exposing apparatus having a supporting mechanism for supporting a sensitive sheet and scanning units, more particularly optical scanning units, to perform one or more of the above-mentioned scanning methods. The particular construction of such a device will be clear from the preferred embodiments that will be described herein after with reference to the drawings.